testing.cffi0.test_ffi_backend.TestFFI fails on glibc-2.34 and above

glibc-2.34 merged libdl.so into libc.so: https://sourceware.org/pipermail/libc-alpha/2021-August/129718.html

* In order to support smoother in-place-upgrades and to simplify
  the implementation of the runtime all functionality formerly
  implemented in the libraries libpthread, libdl, libutil, libanl has
  been integrated into libc.  New applications do not need to link with
  -lpthread, -ldl, -lutil, -lanl anymore.  For backwards compatibility,
  empty static archives libpthread.a, libdl.a, libutil.a, libanl.a are
  provided, so that the linker options keep working.

Unfortunately it breaks dlopen("dl"):

=========================== short test summary info ============================
FAILED testing/cffi0/test_ffi_backend.py::TestFFI::test_dlopen_handle - OSErr...
FAILED testing/cffi1/test_re_python.py::test_dlopen_handle - OSError: ctypes....
...
__________________________ TestFFI.test_dlopen_handle __________________________
E           OSError: ctypes.util.find_library() did not manage to locate a library called 'dl'